1. Field of the Invention
The present invention relates generally to heat exchangers and methods of constructing heat exchangers.
2. Description of the Related Art
Heat exchangers and heat exchange chemical reactors having large arrays of parallel tubes are used for a variety of industrial processes for transferring heat to a substance without directly exposing the substance to a heat source such as a flame or electrical element. Some heat exchangers are designed in a “tube and shell” arrangement with tubes disposed within a larger container called a shell. A first fluid flows within the tubes. A second fluid, the “shell-side fluid”, such as a gas or liquid, is disposed within the shell, but outside the tubes. The first fluid either absorbs heat from or transfers heat to the second fluid as the first fluid passes in one end of the tubes and out the other.
Some tube and shell heat exchangers include various zones in which the intended heat transfer differs from other zones. In one example, an upstream or intermediate zone provides a first amount of heat transfer by using the second high (low) temperature fluid on the shell side, and another zone, perhaps downstream, provides a different amount of heat transfer by using a third high (low) temperature fluid on the shell side.
Heat exchange chemical reactors are often employed to carry out chemical reactions where significant quantities of heat must be added or removed from a first reacting fluid to a second heat transfer fluid, which may or may not be reacting. These heat exchange reactors often bear a strong resemblance to simple heat exchangers, but may be provided with additional features such as fixed beds of catalysts, specialized flow path designs, exotic materials and the like.
An example of a reaction conducted in heat exchange chemical reactors is the steam reformation of hydrocarbon feed stocks to produce hydrogen-containing gas mixtures. In this process, a mixture of steam and hydrocarbon is passed through one fluid circuit while a hot fluid, usually combustion product, flows through a separate fluid circuit and transfers heat into the reacting first fluid to promote the highly endothermic steam reforming reaction. An example of a plate-fin type hydrocarbon steam reformer is shown in U.S. Pat. No. 5,733,347 to Lesieur. Several examples of tubular heat exchange reformers have been revealed, for example U.S. Pat. No. 3,446,594 to Buswell et al. An advanced tubular reformer configuration which offers significant advantages over other configurations is described in U.S. Pat. Nos. 6,497,856, 6,896,041, 6,957,695, 6,896,041, and 7,117,934 to Lomax, et al., which are incorporated herein by reference in their entirety.
The present inventors have determined that many heat exchange reactors face a serious mechanical design challenge due to the temperature differences between the reacting first fluid and the second heat transfer fluid or from one temperature zone in the heat exchange reactor to the next temperature zone. These temperature differences create thermal strains, or displacements, due to differential expansion of the material of construction of the heat exchange reactor.
The thermal stresses are particularly acute in hydrocarbon steam reformers because the temperature gradients are generally very high. Further, modern heat exchange reactors for steam reforming are designed to reduce the physical size of the reactor to reduce cost and facilitate their employment in space and weight sensitive applications such as vehicles. The reduction in physical size results in an aggravation of the problem of thermal stresses by drastically decreasing the distance across which the thermal gradients occur.
In tubular heat exchange reactors in general, and in the improved reactor of U.S. Pat. No. 6,497,856 in particular, one route to achieving a more compact reactor is the provision of baffles to induce flow of the second fluid in a direction substantially normal or perpendicular to the axis of the tubes. Such a flow arrangement is termed “cross-flow.” By placing several baffle features along the length of the heat exchange reactor tubes, the second heat exchange fluid may be induced to flow across the tube array several times. Through optimal selection of the number and spacing of baffles, the mechanical configuration of a tubular heat exchange reactor may be optimized for factors such as physical size, second fluid pressure drop, and other important features.
Because of differences in the thermal expansion of the tubes disposed within the baffles, expansion of the baffles themselves, and expansion of the support structure during heating, certain baffles have a tendency to push the tubes laterally (parallel to the long axes of the baffle) thus distorting the heat exchanger. In order to remedy this distortion problem, U.S. Pat. No. 7,117,934 incorporates a plurality of holes with varied shapes, increased size, and/or offset centers from the tubes disposed within the baffles. The varied shapes, increased size, and/or offset centers allow for different amounts of thermal expansion between the baffle plate and tubes while reducing or preventing lateral distortion of the tubes by the baffle plate.
The use of holes with varied shapes, increased size, and/or offset centers from the tubes disposed within the baffles may introduce leakage of the second fluid in a direction parallel to the longitudinal axis of the tubes. In other words, while the baffles still cause most of the flow of the second fluid in a direction perpendicular to the tubes (cross-flow), the offset or oversize holes in the baffles allow some of the second fluid to travel along the outer surface of the tubes to traverse the baffle in a direction parallel to the longitudinal axis of the tubes.
Fins may be attached to the tubes to increase the heat transfer rate from the tubes to the shell-side fluid, but depending on the manner in which the fins are attached to the tubes, the fins may not prevent or sufficiently reduce the leakage in the longitudinal direction of the tubes. For example, to save cost, the fins may not be welded to the tubes, but instead press-fit or shrink-fit to the tubes. Therefore, the fins are not integrally attached to the tubes and remain separate components with a possible open seam or gap allowing leakage in the longitudinal direction between the fins and tubes.
The above-noted leakage along the longitudinal axis of the tubes can reduce the effectiveness of the zones in maintaining different temperatures of second fluid. For example, as noted above, in some cases, an upstream or intermediate zone uses a high temperature fluid on the shell side, and a downstream zone uses a lower temperature fluid on the shell side.
FIGS. 1a and 1b show one example of a conventional heat exchange reactor. FIG. 1b shows the core 101 of a reactor with a second pair of cross-flow fluid passageways 109, which is on a shell side of the array of tubes 2 and is typically operated at a temperature different, e.g., lower temperature, than a first serial array of cross-flow fluid passageways 108, which is also on a shell side of the array of tubes 2. Thus, the first and second cross-flow passageways comprise first and second temperature zones.
If leakage occurs along the direction of the longitudinal axis of the tubes, then the lower temperature fluid in the second cross-flow fluid passageway 109 can leak into the higher temperature first cross-flow fluid passageway and have a deleterious cooling effect on the upstream zone. This problem is even more undesirable if the second fluid in the upstream zone has a different chemical composition than the second fluid in the downstream zone.
In order to prevent or reduce the above-noted leakage of second fluid between the upstream and downstream zones, U.S. Pat. No. 6,957,695 provides a series of refractory sheets, which can be, for example, a felt material. FIG. 2 represents an embodiment of the apparatus described in U.S. Pat. No. 6,957,695.
As shown in FIG. 2, the refractory felt seal 16 is stacked between the first or upper fluid passage 8, which is a high temperature zone at a first pressure, and a second or lower fluid passage 9, which is a relatively low temperature zone at a second, higher pressure. Refractory felt seal 16 sometimes allows excessive leakage along the outer surface of the tubes, which causes an undesirable and unintended rapid cooling of the array of tubes 2. This undesirable cooling is especially deleterious if the heat exchanger is a steam reforming heat exchange reactor, as it causes a non-linear impact on the thermodynamic limits to conversion of the hydrocarbon reactant.
Background FIG. 2 is a side section view of the heat exchanger sealing zone 7 of a background heat exchange reactor as described in U.S. Pat. No. 6,957,695. The sealing zone 7 is defined by baffle plates 13 and 15. FIG. 2 shows an array of substantially parallel tubes 2 with the associated plate fins 10. Cover plates 30 are also visible and are joined to the extended baffle plate 15 and the full baffle plate 13.
The baffle plates have local gaps between surfaces of the holes therethrough and the tubes of the tube array 2 that pass through the holes. Additional gaps 50 may exist between refractory felt seals 16 and the cover pan wall within the sealing zone 7. The gaps, which are typically provided by oversizing tube holes in the baffles, create fluid leak paths which lead to fluid leakage between the first cross-flow fluid passageway 8 and the second cross-flow fluid passageway 9. These two passageways may convey the same fluid or two different fluids, but in either case it is likely that a pressure differential will exist between the fluid passages, and leakage between the two passages will result. In certain configurations, the first cross-flow fluid passageway 8 contains a high temperature burner flue gas at a first pressure, while the second fluid passage 9 contains preheated burner air at a second, higher pressure. In this case, the refractory felt seals 16 would function to reduce leakage and thermal stresses, but leakage in the direction of the longitudinal axis of the tubes in the tube array 2 may still occur.
A sealing zone 7 including a refractory felt material as described above is especially important when the fluid in passageway 8 is at a temperature above a service limit for intumescent material of 800° C., which is sometimes used in concert with the felt material, and the fluid in passageway 9 is below the service limit for the intumescent material. The gap between the baffle plates 13 and 15 is filled with one or more layers of the refractory felt gaskets 16. One refractory felt seal 16 is in direct contact with the baffle 15, which is in contact with the fluid passageway 9. This refractory material is initially installed in sealing contact with the tubes of the tube array 2, the baffle 15, and the internal surface of the housing 100. One or more layers of intumescent material 56, which are depicted by dashed lines in FIG. 2, are then provided between the refractory material 16 and the baffle 13. The intumescent material 56 is separated from the fluid passage 8 by refractory felt seals 16, which are intended to act not only as seals, but also as a thermal insulator to prevent overheating of the intumescent material 56. The two baffles are held in essentially fixed mechanical relationship by mechanical means such as connection to baffle support rods as known in the art, by mechanical capture between layers of extended heat exchange fins in intimate contact with the array of tubes 2.
Upon heating above 300° C., the intumescent material 56 expands in a direction normal to the face of the baffles 13, 15. This expansion subjects the refractory felt seals 16 to substantial pressure, which is intended to improve their sealing effect. The gap between the substantially-parallel plates 13, 54 is filled with an intumescent material 55, which expands at elevated temperatures. This intumescent material is unique in its ability to expand at temperatures between 300° C. and 375° C., and to remain elastic at temperatures as high as 800° C. for extended exposure. Intumescent material has the property of expanding, when heated, much more noticeably in a direction normal to its thickness than in a direction parallel to its thickness. Therefore, its use as a sealing member alone or even as a primary sealing member was thought to be insufficient in a tubular array heat exchanger of the type contemplated here, and conventional heat exchanger or reactor designs incorporating the sealing zone 7 relied heavily on the refractory felt gaskets 16 instead.
As can be seen in FIG. 2, the walls 34 connect the bottom portion of the first cross-flow fluid passageway 8 and the second cross-flow fluid passageway 9. In the above-noted example, the second fluid in the first cross-flow fluid passageway 8 is intended to be at a temperature of 800° C. or higher, and the second fluid in the second cross-flow fluid passageway 9 is intended to be at a temperature of 300° C. or higher. Accordingly, the structure surrounding the first cross-flow fluid passageway 8 typically expands more during processing than does the structure surrounding the second cross-flow fluid passageway 9. Because of this difference in expansion during processing, the housing chamber 30 often warps or bows after processing so as to form a “smile” shape after processing is finished and the system has cooled.
It is believed that this bowed shape is caused by transfer of force from the structure surrounding the first cross-flow fluid passageway 8 to the second cross-flow fluid passageway 9 via the walls 34. As the structure proximate to the second fluid passage way, e.g., the extended baffle plate 15, expands during heating, the force of this expansion is transferred to the full baffle 13, which is not as hot during processing as the extended baffle plate 15. The full baffle 13 then undergoes plastic deformation. After the process is completed, the system cools, and the extended baffle 15 contracts, not yet having undergone plastic deformation or at least not to the same extent as the full baffle plate 13. Due to plastic deformation, the full baffle 13 is often longer than it was originally and causes the “smile” shape noted above. This process may then cause crinkling of the extended baffle 15.
The above-noted deformations can exacerbate the leakage issue discussed above inasmuch as the crinkling of the extended baffle plate 15 and bowing of the full baffle 13 can cause or increase misalignment of the array of tubes 2 and the holes disposed in the baffle plates.
Accordingly, it is desired to provide a heat exchanger or reactor that can provide at least two different zones of heat with an isolation or insulation zone between the different heat zones that more effectively reduces leakage along the direction of the longitudinal axes of the tubes.
Additionally, it is desired to provide a heat exchanger or reactor that reduces the transfer of mechanical forces from one heat zone to the other heat zone.